A Planning Approach to Population and Employment Forecasting
It’s the age old question: “What comes first, the chicken or the egg?” In long range planning, that question gets transformed to: “Does growth dictate the land use or does land use guide the growth?” This article explores a forecasting methodology that incorporates both responses.
As part of the update to the 2035 Long Range Transportation Plan (LRTP), population and employment forecasts for the years 2025 and 2035 are needed. The LRTP serves as a guide for local multi-modal transportation priorities and investments over the next twenty-plus years. The population and employment forecasts are a principal input into the transportation modeling process which is used to identify transportation needs. Forecasting countywide data twenty years into the future is hard enough, but the LRTP requires forecasts by traffic analysis zone (TAZ). TAZs are small geographic areas representing areas of travel activity and are delineated specifically for analyzing transportation. Hillsborough 
County has 758 TAZs, making this a staggering task.
Traditionally population and employment forecasting was conducted by reviewing the past. Based on historical trend of the past twenty years, the “next” twenty years can be projected. This is equivalent to believing growth dictates the land use. Although this “trend” method is acceptable, it may not be the best method for long range planning purposes. A comprehensive plan is a twenty-year blueprint for future growth. It is a legal document, adopted by local governments, to guide and manage short and long range growth and development. Trend analyses ignore the influence of growth management and comprehensive plan policies and strategies by projecting the future based upon the past.
Therefore a new forecasting approach was developed. This method reflects the longer-term intent to base infrastructure investment decisions on the goals and policies of the local comprehensive plans of Hillsborough County and the cities of Tampa, Temple Terrace, and Plant City. For example, a policy to accommodate most growth within the Urban Service Area (USA) boundary, with the 
remaining growth targeted to Plant City and areas outside of the USA where development exists, influenced the forecast. Other strategies included focusing growth in activity centers and in designated transportation corridors. Incorporating these strategies meant purposely shifting population and employment among the TAZs. In other words, land use is guiding growth.
Historical trends were not completely discarded. In an effort to reflect both existing (and somewhat entrenched) development patterns, a small percentage of new growth was allocated among the TAZs based on the past. The trend allocation is based upon the existing share of population or employment currently found within a TAZ. Incorporating the “trend” helps to capture the existing development pattern which, while perhaps not desirable, does reflect the reality that development patterns can be altered only slowly over time.
